1. Field of the Invention
The present invention relates to the field of gas turbine engines and more particularly, turbojet engines. It more particularly relates to the lubrication in an air starter mounted on the power transmission gear box.
2. Description of Related Art
A turbojet engine comprises a housing fixed on the external casing and wherein there are arranged pinions being mechanically connected with an engine shaft to be driven thereby. They are, in turn, connected with machines for auxiliary engines such as pumps or electrical generators supplying the engine. More particularly, such a housing supports the starter that, especially at the ground start, for example mechanically drives the engine shaft, the one of the high pressure rotor in a multi-body engine. The housing is commonly referred to as AGB which is the acronym for the words “Auxiliary Gear Box”. Lubricating rotary parts inside the starter casing generally occurs by means of oil taken from the AGB housing. FIG. 1 schematically illustrates a starter lubricating mode of the starter, such as implemented on some turbojet engines. A starter 1 is mounted on the AGB housing 2. A recess-forming interface 3 is arranged between the casing wall 4 and the wall of the AGB housing 2. The starter shaft 5 together with its pinion assembly is supported in the starter casing by a bearing 6. On the left-hand side of FIG. 1 and not shown, the shaft 5 is connected with the air turbine for its driving thereof at the engine start. A turbine seal 5A provides for the tightness of the shaft 5 which crosses the casing at this point.
The lubricating oil of rotary parts of the starter is supplied from the housing 2 into a receptacle 7 located in the interface 3 between the starter and the AGB housing and which is integral with the starter. Such a receptacle 7 communicates through an aperture 8, arranged in the wall 4 of the starter casing, with the enclosure of the latter. The aperture is protected by a strainer for preventing small fragments from being projected into the starter recess. The oil collected in the lower part of the starter oil enclosure is circulated across the rotary parts of the starter through sparging, pumping or any other means, not shown. There is no oil circulation between the enclosure of the starter and that of the housing 2. The level is defined by an overflow 7′ of the receptacle 7 of the interface 3. Pressure balance between both enclosures, for the starter and for the AGB, is provided by an aperture 8′ arranged in the upper part of the wall 4. Through such a communicating vessel principle, the monitoring of the starter oil level is associated with the monitoring in real time of the engine oil circuit. Thereby, both the clogging of the starter and the risk for contamination of the engine by the oil from the starter are prevented.
The oil contained in the starter enclosure may inadvertently leak outwards for various reasons. The seal 5a between the air turbine shaft and the starter casing may have been damaged; for example, an oil plug may no longer be tight or even, a crack in the casing may have occurred. Should such an event occur, the leakage has no consequence on the starter lubrication through oil supply from the AGB housing across the strainer. However, as the pressure being present in the enclosures is higher than the atmospheric pressure, the air loaded with oil mist from the AGB housing is expelled outwards. Such a supply is then considered as a leakage of the engine lubricating oil and is sent to the pilot. The leakage may cause the engine lubricating oil circuit to be purged, and hence, the engine could be cut-off in flight (IFSD).
A solution disclosed in the U.S. Pat. No. 6,681,579 involves checking the fluid flow between the AGB housing and the starter enclosure. A sealing plate is mounted on the starter on the side of the interface-forming area between the starter and the housing so as to make it possible to insulate the starter enclosure when the pressure inside the latter drops. A valve remains opened in normal operation so as to ensure the pressure balance. Such a valve automatically closes should the pressure drop.